1. Field of the Invention
This invention relates to an imaging optical system using distributed index optical elements, and in particular to an imaging optical system suitable for use as the optical system of a business machine such as a copying apparatus or a facsimile apparatus.
2. Description of the Prior Art
In the optical system of a business machine such as a copying apparatus or a facsimile apparatus, a double-eye optical system of erect one-to-one magnification such as Selfoc (trade name of a product of Japan Plate Glass Co., Ltd.) has heretofore been used as the imaging optical system instead of a single spherical lens system, whereby compactness of the apparatus has been achieved. Where such distributed index optical elements, i.e., so-called radial distributed index optical elements having an index distribution symmetrical about the optic axis in the radial direction, are arranged in an array-like form, two rows of distributed index optical element arrays are juxtaposed proximately to each other in staggered relationship as shown in Japanese Utility Model Publication No. 9230/1984 in order to eliminate the irregularity of light amount on the image plane. However, the use of plural rows of arrays leads to an increased number of optical elements and therefore to the expensiveness of the apparatus, and further requires the accuracy of arrangement to be very high, and this in turn leads to the difficulties in manufacturing.
For this reason, it has heretofore been desired to arrange a plurality of distributed index optical elements in a row to thereby prepare an imaging optical system of erect one-to-one magnification, and a report of studies on the irregularity of light amount when such an imaging optical system is used is made in Optics, Vol. 10, No. 2, p. 111, April 1981. However, the imaging optical systems of a construction in which distributed index optical elements are arranged in a row as heretofore disclosed, including the above-mentioned report, have been no better than mere studies and have been far from being put into practical use. This is because, when, for example, such an imaging optical system is to be put into practical use, forming the optical elements in a row gives rise to the problem of insufficiency of light amount in addition to the irregularity of light amount.